The prior art shows methods whereby a signal indicative of the level of a non-conductive material within a vessel may be measured by, for example, a capacitive probe. Moreover, the prior art shows methods whereby the signal may be compensated by a second signal indicative of the dielectric constant of the material being measured whereby the signal is indicative of the actual level of the material, even if the dielectric constant of the material changes from time to time. Such a corrected signal will be referred to hereinafter as a compensated signal. See, e.g., commonly assigned U.S. Pat. Nos. 4,232,300, 4,235,106 and 4,208,909, issued to Wright et al and Maltby et al, respectively.
The prior art also shows systems in which signals of varying types are generated at a first location by an instrument and transmitted to a remote location for control purposes. In particular the prior art shows methods whereby the power used by the instrument in generation of the signal is transmitted over the same wires as is the signal itself. Two wires are generally adequate to achieve this purpose and hence these systems are known as two-wire systems. However, there has as yet been provided no two-wire true level measurement instrument.
As part of level measurement instrument systems there have been provided circuits for dividing one signal into another, e.g., for using a compensating signal indicative of the dielectric constant of a material to be measured as the denominator of a fraction, the numerator of which is the signal indicative of the level of the material. The quotient or result of the division is then the true level of the material. However, such division circuits as proposed in the prior art, for example, in Scott U.S. Pat. No. 3,981,586 have involved excess circuit complexity which would desirably be avoided. Furthermore, it will be appreciated by those skilled in the art that convention dictates that the signal levels passing over a two-wire current system be defined to be between 4 and 20 milliamperes (ma). Accordingly, the power drawn by the instrument cannot be greater than 4 ma. Typical prior art divider circuits involve several amplification stages which tend to draw more than this maximum current. For example, U.S. Pat. No. 3,024,999 to Hickock, Jr., requires two ramp generators, while the Scott patent referred to above requires dual integrators.
Other prior art divider circuits, such as shown in U.S. Pat. No. 3,493,738 to Keller require duplicate circuitry for holding of reference and sample values, for example, dual resistor-capacitor networks. It will be apparent to those skilled in the art that the use of such paralleled components limits the accuracy of the system to the degree of identity of the values of the components, and that furthermore these values may vary in ways different from one another over time, such that the accuracy of the circuit is likely to deteriorate with time.
Another prior art ratio measuring scheme is disclosed in U.S. Pat. No. 3,652,930 to Sugiyama et al. This circuit requires an actual multiplication of two functions, which is difficult to implement inexpensively and accurately, and furthermore requires a theoretical approximation which may not be true in all cases.